Now scientists have shown that it may be more than just a by-product of fear.

Tests on rats suggest signals sent from the gut to the brain actually drive key elements of the fear response.

When the link was severed, animals became less wary of open spaces and bright lights.

"The innate response to fear appears to be influenced significantly by signals sent from the stomach to the brain," lead researcher Dr Urs Meyer, from the Swiss science institute ETH Zurich, said.

A branch of the vagus nerve carries messages from the gut to the brain, while another branch takes them in the opposite direction.

Cutting the former, known as the afferent vagus nerve, directly affected the rats' instinctive fear response.

But it had no impact on "learned" fear.

Innate and conditioned fear involve different signalling systems in the brain, the scientists pointed out.

Closer investigation showed that the loss of signals from the abdomen altered the production of certain neurotransmitter messaging chemicals in the brain.

"We were able to show for the first time that the selective interruption of the signal path from the stomach to the brain changed complex behavioural patterns," Dr Meyer said.

"This has traditionally been attributed to the brain alone."

Precisely what the stomach is "saying" to the brain is not yet clear, according to the researchers whose findings appear in the Journal of Neuroscience.

They believe the work might have implications for treating people with Post Traumatic Stress Disorder.

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